The central lymphoid tissues undergo significant atrophy commencing at puberty. This causes decreased T cell responsiveness to mitogens and may decrease T cell suppressor activity. Either event may hasten aging of the entire organism. Several experiments have demonstrated significant correlations between central lymphoid organ size and thyroxine concentration. This suggests that appropriate doses of thyroxine administration might retard aging of the central lymphoid system. In addition, thryroxine administration might retard development of autoimmune thyroiditis in the Obese Strain (OS) chickens, a strain characterized by hypothyroidism at hatching as well as a significantly decreased size of the thymus and bursa of Fabricius. This research proposal outlines experiments in which various levels of thyroxine are administered to OS embryos and newly hatched chicks. The effects of this treatment on thyroiditis and central lymphoid organ development are assessed. In addition, the effects of thyroxine administration in preventing the atrophy of the central lymphoid tissues of normal chickens is also determined.